I have very WIPpy support for the charset (only) currently in C64Studio. It’s just enough so I can write my game entry. This means character set (FCM mode) and character screen, plus the obligatory C64 hires, MC and ECM modes.
If you’re feeling adventurous look at https://www.georg-rottensteiner.de/webmisc/C64StudioRelease.zip
And yes, it’s really WIP. Some things may not work properly.